1. Field of the Invention
The present invention relates to motion sensing, and more specifically, to an optical device for sensing the position or movement of an object.
2. Discussion of the Prior Art
Anti-lock braking systems (ABS) are employed on automobiles and aircraft. Wheel speed sensors are an integral part of ABS systems. In a typical construction, an inductive field proximity sensor generates a pulse train in response to a toothed metallic disk attached to the rotating wheel. However, hostile environmental conditions, such as high temperature and electromagnetic interference, cause problems for inductive devices. Therefore, fiber optic tachometers have recently become popular for use as wheel speed sensors.
One approach to the design of optical fiber wheel speed sensors uses two fibers. The first fiber carries the light from a remote emitter to the wheel disk, where it is then chopped or modulated, received by the second fiber, and returned via the second fiber to a photodetector unit where the optical signal is converted to an electrical signal and processed electronically into the desired output format. The modulation causes the signal to vary from high level (light received by second fiber) to low level (light blocked from second fiber).
Another approach to optical fiber wheel speed sensor design uses a single multimode fiber. See U.S. Pat. No. 4,767,164. The fiber carries light from a remote emitter to the wheel, where a rotatable interrupter, coupled to the wheel, has reflective elements spaced apart on the interrupter to reflect the light back into the fiber. The reflected light is returned in the fiber to a photodetection circuit where a high level signal indicative of operation is generated. When light strikes the rotating interrupter at a position between the reflective elements, no light is reflected and the signal generated by the photodetection circuit represents a low level.
It is highly desirable for ABS and other sensing systems to have self-test functions to verify proper operation. One aspect of the self-test should provide a positive indication that the sensors are connected and operable. This self-test interrogation is often called "built-in-test" (BIT).
Since the light source is typically located remotely from the speed sensor, several optical fibers must be linked together by optical connectors to reach the sensor location. However, optical connectors inherently reflect some of the source light back up the fiber. This creates a background noise level which makes it difficult to differentiate between the noise level and the low level return signal from the speed sensor. This background noise is particularly troublesome when a built-in-test is employed to determine whether the optical path is intact and operating properly. In prior designs, the signal being returned to the photodetector unit varied between a high and a low state. The magnitude of the low state was often of the same magnitude as the background noise caused by reflections off the optical connections in the path. It is, therefore, highly desirable to have a system in which the low level signals being returned from the wheel disk are easily distinguishable from the background noise.